[llvm-dev] on division of __int128 bit integer

[Paweł Bylica via llvm-dev]

Hi,

First of all, the code in compiler-rt/builtins is rather confusing for me.
No idea what `tu_int` is but I'm guessing this must be `unsigned __int128`,
not `unsigned long long`.
https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/builtins/udivmodti4.c#L22

Secondly the "slow" case proceeds single bit at a time, so this loop may
need ~64 iterations in worst case:
https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/builtins/udivmodti4.c#L167-L188

>From measurements, it looks GCC has it implemented more efficiently
although I have no idea where do they have the code for it.

If you are interested, I have division implemented for unsigned 128-bit
integer here:
https://github.com/chfast/intx/blob/master/include/intx/int128.hpp#L632-L687
>From quick check it takes 16 ns, GCC runtime lib: 25 ns, GMP: 20 ns.

It follows "Improved division by invariant integers"
https://gmplib.org/~tege/division-paper.pdf

If make sense I can contribute this implementation as replacement for
`__udivmodti4`. Although I'm not sure what requirements for __udivmodti4
are.

--
Paweł


On Thu, May 21, 2020 at 11:30 AM Vivek Pandey via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Team,
>
> I observer that division of __int128 bit is very heavy operation.
> It internally call a routine '__udivti3', which internally call '
> __udivmodti4'.
>
> Due to it the overall performance is much much slower (almost 15 time
> slower than if I do it via a combination of 64-bit or microsoft '_udiv128
> ').
>
> Also what to know if I can directly call below routine directly from my
> code :
> *unsigned long long* *__udivmodti4* *(unsigned long long *a*, unsigned
> long long *b*, unsigned long long **c*)*
>
> This is against statement  "Conceptually,in operation, like ‘/’ or ‘%’,
> no function is called, so no header is provided for *__udivmodti4* etc. –
> and we should not call them explicitly in our code." but wanted to know
> more on it.
>
> Thank you!
> Regards,
> Vivek JP Pandey,
> Software Professional,
> *+91-7775054441*
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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>
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